In recent years, in order to meet a demand for enhancement of fuel efficiency and improvement of safety in automobiles, high-strength thin steel sheets have come to be used for automobile bodies, and it is required to weld these steel sheets by using laser welding. Further, in a method of welding the high-strength stacked thin steel sheets, a laser welding method with which stable strength of joint portions can be obtained is desired.
The laser welding uses a laser beam as a heat source and therefore can surely and easily control an input heat amount as compared with arc welding such as TIG welding and MIG welding. Accordingly, it is possible to reduce thermal deformation by appropriately setting welding conditions such as a welding speed and radiation output of the laser beam and further a flow rate of shielding gas and so on. Further, the laser welding is capable of welding from one side and thus is suitable for assembly welding of complicated members such as automobile bodies.
Actually, the laser welding is often adopted for welding members molded from thin steel sheets in the automobile manufacturing industry, the electric equipment manufacturing industry, and other fields. Further, relating to this, there has been proposed a laser welding method of a lap joint excellent in weld joint strength.
Patent Literature 1, for instance, discloses a method which realizes quality improvement by tempering a first bead by heat of a second bead to prevent the beads from being easily fractured at the time of the molding, thereby improving moldability.